124 PRACTICAL PAEASITOLOGY 



containing larvae of A. duodenale. Upon another occasion, a drop of 

 water containing larvae was spread upon the skin of a living limb 

 shortly before amputation, and the portion of skin which had been 

 wetted was examined immediately after the operation. This experi- 

 ment, though offering conclusive proof -of penetration only, showed 

 very distinctly the manner in which this was effected, the larvae 

 entering mainly through the hair-follicles and shedding their skin in 

 the process. The experiments which Looss performed upon himself 

 proved, not only penetration but infection also ; that is, worms were 

 subsequently found to be present in the bowel. These results have 

 been confirmed by other authors and may be verified by means of a 

 simple experiment. All that is necessary is to bring water or earth 

 containing larvae of A. trigonoceplialum into contact with the skin of 

 a dog, and to keep the place covered with a bandage for one to two 

 hours. The larvae will penetrate the skin of young dogs more readily 

 and in larger numbers than that of fall-grown dogs. For this reason 

 puppies frequently die, while older dogs bear infection very well. 

 Upon examination of the intestine, however, comparatively few worms 

 are to be found, the majority appearing to perish in the course of their 

 wanderings. It will, of course, be understood that when introduced 

 by the mouth, the parasites attain sexual maturity more quickly than 

 when introduced by the skin. The larvae do not ripen except in the 

 bowel and, in the latter case, they only reach it after a long journey. 



The Cestodes, the digenetic Trematodes, and certain of the 

 Nematodes require the agency of two, and sometimes three, distinct 

 host-species in order to complete their life-history. There is, first, 

 the definitive host, almost invariably a mammalian, in which the 

 worm attains full size and becomes sexually mature ; and second, 

 the intermediate host, in which the embryo or larva develops into 

 an encysted intermediate stage, either directly or after asexual 

 multiplication. 



Tcenia crassicollis of cats is a Cestode eminently suited for 

 experimental purposes. The Cysticercus stage is passed in the liver 

 of rats and mice. The parasites may be reared in the following 

 manner. The mature eggs, which are formed in vast numbers, are 

 obtained by pricking the uterus of a ripe proglottis from the cat. 

 They are transferred to bread soaked in milk and the mass is used 

 to feed white mice. If the eggs are mature and the mice healthy, 

 the results are almost certain. The dose should be carefully 

 regulated, however, as if too many of the eggs are swallowed the 

 liver becomes very much changed and the mice will die in the course 

 of the second week. 



As early as five hours after infection, free oncospheres will be 

 found in, generally, the middle third of the small intestine. At the 



